1. V. D. Belousov,Systems of Quasigroups with Generalized Identities (Russian), Uspehi Mat. Nauk20, #1 (121), 75–146 (1965); English translation in Russian Math. Surveys20, #1, 75–143 (1965), in part. pp. 79, 80, 85, 86, 88.
2. G. Grätzer,Universal Algebra (Van Nostrand, Princeton N.J.-Toronto-London-Melbourne 1968), in part. §§1, 8.